A strong demand has arisen for a magnetic recording medium having a higher recording density. One approach to meet this demand is to make the surface of the magnetic layer smooth. However, when the surface of the magnetic layer is made smooth, the friction coefficient between the magnetic layer and the tape running system increases as the magnetic recording medium runs. This results in the magnetic layer being easily damaged or the magnetic layer being easily peeled off in a short period of its use.
In the case of a video tape, the magnetic layer is put under harsh conditions, such as during a still mode. Under such harsh conditions, ferromagnetic particles easily come off from the magnetic layer. This causes the magnetic head gap bridging (head clogging).
Conventionally, abrasive agents (hard particles) such as corundum, silicon carbide, chromium oxide, etc. are added to the magnetic layer to improve the running durability of the magnetic layer. In the case when abrasive agents are added to the magnetic layer to improve the running durability of the magnetic layer, a comparatively large amount thereof must be added to exhibit its abrasive effects However, the addition of such large amounts of the abrasive agents to the magnetic layer causes great wear on the magnetic head. Moreover, such is unfavorable for smoothing the surface of the magnetic layer so as to improve the electromagnetic properties.
It is also proposed that a fatty acid or an ester of a fatty acid and an aliphatic alcohol is added as a lubricating agent to the magnetic layer to reduce the friction coefficient.
With the recent increasing usage of portable video tape recorders and flexible disk drive apparatuses for personal computers, the magnetic recording medium is expected to be used under various conditions such as at a low temperature or at a high temperature and a high humidity. Accordingly, the running durability of the magnetic recording medium must be stable so as not to change under various conceivable conditions. The above-described conventional lubricating agents are not satisfactory for this purpose.
In video tapes and floppy disks, as the size of the magnetic recording medium is minimized by shortening the recording wavelength, as well as the track width, ferromagnetic alloy particles are more increasingly used than iron oxide type ferromagnetic particles as a material for the magnetic composition, and those having a smaller particles size are increasingly used. In this way, relatively excellent electromagnetic properties can be obtained by radically minimizing the size of the magnetic particles, but it is difficult to simultaneously achieve excellent running durability.
As a result of investigations as to lubricating agents to avoid the above-described disadvantages, it was found that stable running durability, which is stable even under harsh conditions such as at a high temperature and a high humidity or at a low temperature and a low humidity, can be obtained by incorporating an alkane sulfonate or an alkyl sulfate into a magnetic layer. (U.S. patent application Ser. No. 033,704 filed on Apr. 3, 1987). However, an alkane sulfonate or an alkyl sulfate generally have a low solubility in an organic solvent and therefore crystals are deposited on the magnetic layer. This causes video head gap bridging, particularly at a low humidity.
Examples of the magnetic recording medium containing an alkane sulfonate as a lubricating agent are described in Japanese Patent Publication No. 12949/72, but the characteristics thereof are not fully satisfactory.